1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus which performs image formation by developing a latent image formed on a photosensitive member using a laser beam, and then transferring the developed image onto a recording sheet, and a method of controlling the image forming apparatus.
2. Description of the Related Art
Conventionally, there has been proposed an image forming apparatus including an image forming section shown in FIGS. 13 and 14, which performs image formation by irradiating a photosensitive member with a laser beam to form a visible image, and then transferring the image onto a recording sheet. A rotary polygon mirror 201 includes four reflecting surfaces (each of which is provided for scanning one line), and is driven for rotation by a polygon motor (laser scanner motor) 202 in a direction indicated by an arrow in FIG. 13. A laser diode 203 is turned on or off by a drive circuit (not shown) according to the image signal, and emits an optically modulated laser beam to the rotary polygon mirror 201.
As the polygon mirror 201 rotates, the laser beam is reflected on the reflecting surfaces of the polygon mirror 201 as a deflection beam continuously changing its angle. Then, the laser beam has its distortion aberration corrected by a lens group (not shown), and is reflected on a reflecting mirror 204, for scanning line by line in the main scanning direction of a photosensitive member 205. The photosensitive member 205 is driven for rotation in a direction indicated by an arrow in FIG. 14, and is charged in advance by a electrostatic charger 206. The photosensitive member 205 is sequentially exposed to light by scanning of the laser beam, whereby an electrostatic latent image is formed thereon. A development device 207 develops the electrostatic latent image with toner to form a visible image. A transfer charger 208 transfers the visible image on the photosensitive member onto a recording sheet conveyed in a direction indicated by an arrow 210 in FIG. 14. The recording sheet having the visible image transferred thereon is conveyed to a fixing device 211 to be fixed, and then is discharged from the apparatus.
In this case, a BD sensor 209 is disposed in the vicinity of a scanning start position at a location toward a lateral side of the photosensitive member 205. The BD sensor 209 detects each laser beam reflected on each reflecting surface of the rotary polygon mirror before scanning of each line by the laser beam, and outputs a BD signal. The BD signal detected is used as a scanning start reference signal indicative of the start of scanning in the main scanning direction. The write start position of each line in the main scanning direction is synchronized with reference to the scanning start reference signal.
Further, there also has been proposed an image forming apparatus provided with an image forming section shown in FIGS. 15A and 15B, which forms color images by mixing toners of four colors (yellow: Y, magenta: M, cyan: C, and black: K). In FIGS. 15A and 15B, members which have functions identical to those of the members appearing in FIGS. 13 and 14 are designated by identical reference numerals (Y, M, C, and K added after the respective reference numerals represent yellow, magenta, cyan, and black image forming members, respectively). Photosensitive members 205Y to 205K are driven for rotation in directions indicated by respective associated arrows in FIG. 15B, and photosensitive surfaces of the respective photosensitive members, charged by electrostatic chargers 206Y to 206K are irradiated with the laser beams to have electrostatic latent images formed thereon.
Development devices 207Y to 207K develop the electrostatic latent images formed on the photosensitive members with toner, for visualization of the latent images. Transfer chargers 208Y to 208K transfer the visualized images on the respective photosensitive members onto a recording sheet conveyed in a direction indicated by an arrow 210 in FIG. 15B. In this case, yellow, magenta, cyan, and black images are sequentially transferred onto the recording sheet by the transfer chargers 208Y to 208K in the mentioned order, to thereby form a color image. The fixing device 211 fixes the color image on the recording sheet.
There have been proposed various techniques concerning an image forming technique that performs image formation in the above-described processes (see e.g. Japanese Patent Laid-Open Publication No. 2005-172997). Further, there has been proposed a technique concerning the rotation control of the rotary polygon mirror, which is applied to a change in an image forming speed (processing speed) (see e.g. Japanese Patent Laid-Open Publication No. 2003-11424). This is a technique capable of changing the rotational speed of the polygon motor (or the rotary polygon mirror) when image formation is performed by changing an image forming speed depending on the type of a recording sheet or the type of an image forming mode (a color mode or a monochrome mode).
However, in the image forming apparatus including the above-described conventional image forming section, when the rotational speed of the polygon motor is switched, an image clock that determines the width of each pixel in the main scanning direction is also required to be switched according to the switched rotational speed. States in which image formation is performed by the image forming apparatus at respective different image forming speeds will be described with reference to FIGS. 16A to 18B.
FIGS. 16A and 16B show the relationship between the BD (Beam Detect) signal and the image clock. In an image forming apparatus forming color images, with a view to improving the productivity, an image forming speed for the monochrome printing and an image forming speed for the color printing are sometimes made different from each other. Further, the image forming speed for the color printing is sometimes inferior to (slower than) the image forming speed for the monochrome printing since the image forming speed for the color printing is limited e.g. by the performance of a fixing device. The examples illustrated in FIGS. 16A and 16B show a case in which the FIG. 16A image forming speed for the monochrome printing is higher than the FIG. 16B image forming speed for the color printing appearing.
In the FIG. 16A monochrome printing, there are set the rotational speed of the polygon motor, i.e. the rotation period thereof (the interval of the BD signal in FIGS. 16A and 16B) for realizing pixels with a resolution of 600 dpi in the main scanning direction and a resolution of 600 dpi in the sub-scanning direction, and an image clock. At this time, when the color printing, whose image forming speed is relatively low, is performed, to realize pixels with the resolution of 600 dpi in the sub-scanning direction, it is required to set the rotational speed of the polygon motor, i.e. the rotation period thereof (the time interval between the BD signal) such that the rotational period is made longer than in the monochrome printing, by a time period Δt.
However, since the image clock is determined by a clock set for the monochrome printing, it is impossible to change the clock, so that extra pixels corresponding to the time period Δt are set. As a consequence, the resolution of pixels in the main scanning direction becomes higher than 600 dpi, whereby pixels are formed in which a resolution in the main scanning direction and a resolution in the sub-scanning direction are different.
FIGS. 17A and 17B show states of pixels. FIG. 17A shows a state of pixels formed by monochrome printing. As described above with reference to FIG. 16A, the image clock is determined according to the image forming speed during the monochrome printing, and hence it is possible to realize the resolution of 600 dpi in the main scanning direction, and the resolution of 600 dpi in the sub-scanning direction.
FIG. 17B shows a state of pixels formed by color printing. As described above with reference to FIG. 16B, the resolution of 600 dpi in the sub-scanning direction can be realized by setting the rotational speed of the polygon motor to be lower. At this time, since the image clock is set according to the monochrome printing, a larger number of pixels are printed over the same scanning distance, which makes it possible to form pixels with a higher resolution than 600 dpi in the main scanning direction, that is, pixels each having a width narrower than that of 600 dpi.
FIGS. 18A and 18B show the relationship between a recording sheet and an image area. FIG. 18A shows a state of pixels formed by monochrome printing. When the monochrome printing is performed, an image with the resolution of 600 dpi both in the main scanning direction and the sub-scanning direction can be obtained, and hence it is possible to form a predetermined image area on the recording sheet.
FIG. 18B shows a state of pixels formed by color printing. As shown in FIG. 17B, when the color printing is performed, the image area of the recording sheet is made shorter by a length ΔL than when the monochrome printing performed, since an image obtained has a resolution of higher than 600 dpi in the main scanning direction although it has the resolution of 600 dpi in the sub-scanning direction. In other words, this causes the problem that an image reduced in size in the main scanning direction of the recording sheet is formed, resulting in an increased margin.
Therefore, the image clock that determines the width of each pixel in the main scanning direction and has strict jitter requirements is required to be provided such that a different image clock is used according to the image forming speed. This results in necessity of provision of a switching circuit for switching image clocks or the like, which requires the mounting area to be increased and increases in costs. Further, due to the high frequency of the image clock, it is necessary to take strong countermeasures against undesired noise emission and the like.